Recently, interests in energy storage technology have been gradually increased. As the use of batteries is enlarged to applications for the storage of energy for portable telephones, camcorders, notebook computers, personal computers and electric vehicles, efforts on the research and development of batteries are increasingly embodied. In this view, the field of electrochemical devices receives the greatest attention, and among them, interests in the development of chargeable/dischargeable secondary batteries are focused. More recently, in the development of such batteries, active studies have been conducted to design a novel electrode and battery, which provide an improved capacity density and specific energy.
Among secondary batteries which are now in use, lithium secondary batteries developed in the early 1990s are in the spotlight due to the advantages of higher drive voltages and far greater energy densities than those of conventional batteries, such as Ni-MH, Ni—Cd and sulfuric acid-lead batteries. However, such lithium secondary batteries have safety problems such as ignition and explosion, which result from the use of a non-aqueous electrolyte, and such problems become more severe with an increase in the capacity density of the batteries.
In the non-aqueous electrolyte secondary batteries, a reduction in the safety of the batteries, which occurs when the batteries are continuously charged, becomes a great problem. One of the causes of the reduction in safety is heat generation resulting from the structural degradation of the battery cathode (positive electrode). The heat generation occurs based on the following principles. Specifically, a cathode active material in the non-aqueous electrolyte battery consists of, for example, lithium-containing metal oxide capable of intercalating and deintercalating lithium and/or lithium ions, and such a cathode active material is changed into a thermodynamically unstable structure as a result of the deintercalation of a large amount of lithium ions when the battery is overcharged. When the temperature of the battery in this overcharged state reaches a critical temperature due to external physical impact, for example, exposure to high temperatures, oxygen will be released from the cathode active material having the unstable structure. The released oxygen will cause an exothermic decomposition reaction with, for example, an electrolyte solvent. Particularly, the exothermic decomposition of the electrolyte will be further accelerated by oxygen released from the cathode, which is the positive electrode. Due to such successive exothermic decomposition reactions, the battery will undergo thermal runaway, leading to ignition and explosion.
In attempts to control the above-described ignition or explosion resulting from temperature rise in the battery, many solutions have been suggested, and one example thereof is a method that uses non-aqueous electrolyte additives. As the non-aqueous electrolyte additives, the following additives are known: additives that use redox shuttle mechanisms, for example, bromodimethoxybenzene; and additives that use polymerization mechanisms, for example, alkylbenzene derivatives, such as cyclohexylbenzene (CHB), and biphenyl (BP).
However, the additive undergoing oxidation-reduction cycling (redox shuttle) has a problem in that it is not effective when the charge current of the battery is high. Also, when biphenyl (BP) is used alone as a non-aqueous electrolyte additive, the deterioration in the performance of the battery will necessarily occur due to an increase in the resistance of the battery. In addition, when an alkylbenzene derivative, such as cyclohexylbenzene (CHB), is used as an electrolyte additive, there are problems in that the additive will deteriorate the performance of the battery, since it is partially consumed by reaction even upon the repeated cycling, continuous charge as well as overcharge of the battery. For this reason, there is a continued need to develop a method for improving the safety of the non-aqueous electrolyte secondary battery.